1. Field of the Invention
The present invention relates to a photosensitive material that forms a polymer image by heating or exposure to light, and an image forming method which employs the photosensitive material.
2. Related Background Art
Energies used to form or record an image include light, sound, electricity, magnetism, heat, radiations such as electron rays and X-rays, and chemical energy, among which, in particle, widely used are light, electricity, heat energy, or a combination of any of these.
For example, the image forming method that employs the combination of light energy with chemical energy includes a silver salt photographic process and a method in which a diazo copying paper is used. The method that employs the combination of light energy with electric energy includes an electrophotographic system. Also, the method that utilizes heat energy includes a method in which a thermal recording paper or transfer recording paper is used. On the other hand, known as the method that utilizes electric energy is a method in which an electrostatic recording paper, electrothermal recording paper, or electrosensitive recording paper is used.
Of the above image forming methods, the silver salt photographic process can obtain an image having an high resolution. The silver salt photographic process, however, requires the developing and fixing that uses complicated liquid compositions, and the drying of an image (or a print).
Now, development is energetically made on image forming methods that can form an image through a simple processing.
For example, U.S. Pat. No. 4,629,676 teaches a method in which polymerization reaction under dry (thermal) conditions is caused by the photosensitive reaction of silver halide that acts as a trigger, to form an image comprising a polymer.
This method has the advantage that any complicated wet processing is not required, but has had the disadvantage that the polymer formation rate (i.e., polymerization rate of a polymeric compound) is so low that it takes a long time to form the polymer image. Incidentally, this disadvantage arises presumably because of a reaction intermediate (which functions as a polymerization initiator) formed, in the course of heating, by the reaction between silver produced from silver halide by imagewise exposure and a reducing agent, which intermediate is considered to be so stable and has so low activity as the polymerization initiator that the polymerization reaction can not proceed so rapidly.
On the other hand, to cope with this problem to accelerate the polymerization, Japanese Unexamined Patent Publication No. 62-70836 discloses a method in which a thermal polymerization initiator is used.
This method comprises forming a latent image comprising silver metal produced from silver halide by imagewise exposure, converting, by utilizing a catalytic action of the above silver metal and under heating, a reducing agent into an oxidized product having a polymerization inhibitory power different from that of said reducing agent, thereby producing a difference in the polymerization inhibitory power between the reducing agent and the resulting oxidized product and also causing a thermal polymerization reaction utilizing the thermal polymerization initiator, thus forming a polymer image in accordance with the difference in the polymerization inhibitory power.
This method, however, has been accompanied with the disadvantage that a good contrast can be made with difficulty in the polymer image.
This disadvantage arises presumably because the oxidation-reduction reaction takes place in a latent image portion to form the oxidized product and the polymerization reaction to form the polymer image are allowed to take place in the same heating step, so that these reactions may proceed in a competitive fashion and thus the respective reactions may not proceed in a good efficiency.
Also, the image formation according to this method is very unstable in that, for example, the areas on which the polymer is formed may turn into exposed areas or unexposed areas even because of a slight change in the amount of the reducing agent.
In addition, U.S. Pat. No. 4,649,098 discloses a method in which a reducing agent having a polymerization inhibitory power is brought into an oxidized product by imagewise consumption thereof, that is, imagewise oxidation (at imagewise exposed areas) in the course of the developing of silver halide, and, after imagewise inhibition (at imagewise unexposed areas) of polymerization reaction by the action of the residual reducing agent, light energy is uniformly applied onto the whole area from outside to cause photopolymerization at the part where the reducing agent has been consumed, thus forming a polymer image.
The above method has the advantages that it can achieve a high sensitivity in the writing of a latent image since the silver halide is utilized, and the steps of from the writing for the formation of an image up to the whole areal exposure can be separated in a good efficiency. It, however, is difficult to obtain a polymer image having a sufficient contrast. This is caused for the following reason.
The reducing agent used in the above method is in itself a reducing agent that acts as a polymerization inhibitor and ceases to act as the polymerization inhibitor after the reduction of silver halide. Hence the reducing agent at the imagewise exposed area must be sufficiently converted into the oxidized product, in order that the polymerization may be sufficiently achieved. However, the application of heat energy in a sufficient amount upon carrying out the development, with the intention to sufficiently convert the reducing agent at the imagewise exposed area into the oxidized product, may cause an unauthorized oxidation-reduction reaction at the imagewise unexposed areas. On the other hand, the application of heat energy upon a reduced amount in carrying out the development, with the intention to prevent the oxidation-reduction from taking place at the imagewise unexposed area, may conversely make the oxidation-reduction reaction not to sufficiently proceed at the exposed areas. Since in this instance the imagewise exposed area of an oxidation-reduction image is polymerized with difficulty, the light energy in carrying out the whole areal exposure must be applied in an increased amount. This may cause an unauthorized polymerization at the unexposed areas with increase in the amount thereof, eventually making it impossible to obtain the polymer image with a sufficient contrast.
The polymer image to be formed according to the methods as described above is an image comprising a polymerized area and an unpolymerized area. Aiming at making this polymer image visible and further forming it into a color image, U.S. Pat. No. 4,649,098 and so forth disclose various methods that utilize the difference in physical properties and so forth between the polymerized area and unpolymerized area. For example, proposed are a method in which a treatment is made using a liquid that does not dissolve the polymerized area and dissolves the layer of the unpolymerized area, to dissolve out and remove the unpolymerized area (i.e., etching); a method in which, utilizing a difference in adhesion between the polymerized area and unpolymerized area, a sheet such as a plastic film is adhered and thereafter peeled to separate the polymerized area from unpolymerized area under dry conditions (i.e., peeling-apart); in the case that the polymer image is formed into a color image, a method in which a photopolymerizable layer is previously colored with use of a pigment or dye, which is then subjected to dissolving-out (i.e., the above etching) or peeling (i.e., the above peeling-apart) to form the color image, or a method in which, utilizing the adhesion at the unpolymerized area, a coloring powder is applied to make selective coloring (i.e., toning or inking), or, utilizing a difference in liquid-permeability between the polymerized area and unpolymerized area, the unpolymerized area and selectively dyed by treating it with a dye solution.
However, no polymer image having a sufficient contrast can be obtained in the conventional polymer image forming methods as discussed above, and hence, even with employment of any of the above methods for making the image visible or forming it into a color image, the visible image and color image which are obtained from such a polymer image can not have any sufficient contrast, and particularly it has been difficult to obtain highly detailed visible image and color image.
To cope with the above, the present inventors has ever proposed a photosensitive composition, a photosensitive material, and an image forming method, in which an oxidized product having a polymerization inhibitory power is formed by exposure to light and heating, and thereafter a polymer image is formed at an imagewise unexposed area (U.S. patent application filed Jan. 29, 1989, based on Japanese Patent Application Nos. 63-17155, 63-18502, 63-183441, 63-251958 and 1-1153).